Known contactless power feed equipment is disclosed in Japanese Patent Laid-Open No. 2009-101884.
The contactless power feed equipment includes a continuous induction line provided along the travel path of a mobile unit and a power supply device that supplies a high-frequency current at a predetermined frequency to the induction line.
The power supply device includes: a rectifier that converts the alternating current of an alternating-current power supply (commercial power supply) to a direct current, the alternating-current power supply having a varying alternating voltage; a step-up/down circuit that raises and lowers the output voltage (direct voltage) of the rectifier according to the load of the induction line; and an inverter.
In the inverter, a direct current raised or lowered by the step-up/down circuit is converted into a constant alternating current at the predetermined frequency by a plurality of switching elements driven to keep a constant current flowing to the induction line under PWM control, and then the alternating current is supplied as an output current to the induction line.
The mobile unit includes a power receiving coil opposed to the induction line, the power receiving coil receiving an electromotive force induced from the induction line. The power receiving coil is connected to a power receiving unit that controls an output voltage to a constant voltage and supplies the voltage to a load having varying power consumption.
With this configuration, the constant alternating current at the predetermined frequency is supplied from the power supply device to the induction line, and the mobile unit receives power for the load by an electromotive force induced from the induction line to the power receiving coil. Also in the case where the alternating-current power supply varies in alternating voltage, the step-up/down circuit raises or lowers a direct voltage applied to the inverter, depending on the state of the load of the induction line. Thus, the alternating current passing through the induction line is stably controlled to a constant current.
Japanese Patent Laid-Open No. 11-341713 discloses an example of a secondary-side power receiving circuit having the functions of the power receiving coil and the power receiving unit.
In the secondary-side power receiving circuit, a resonant capacitor is connected to the power receiving coil, the resonant capacitor forming a resonant circuit that resonates at the frequency of an induction line with the power receiving coil. A rectifying circuit is connected to the resonant capacitor and a constant voltage control circuit that controls an output voltage to a reference voltage. A load is connected to the constant voltage control circuit to receive power from the constant voltage control circuit in a resonant condition.
The constant voltage control circuit includes a choke coil, a diode, an output capacitor (voltage capacitor), a switching device (e.g., a transistor for power adjustment) that switches between a connected condition (the switching device is turned on) and an opened condition (the switching device is turned off) across the output ends of the rectifying circuit, and a control circuit.
The control circuit controls the switching device so as to control an output voltage (the voltage of the load, that is, the voltage of the output capacitor) to a reference voltage. In other words, the output voltage is measured, the load decreases while the output voltage (a voltage across the output capacitor) increases, and then the output voltage exceeds a preset reference voltage. At this point, the switching device is placed in the connected condition to reduce the output voltage. When the output voltage returns to the reference voltage, the switching device is placed in the opened condition to keep the output voltage to the reference voltage.
Unfortunately, the known contactless power feed equipment causes the following problems: in an initial condition before the induction line receives a high-frequency current, the constant voltage control circuit of each mobile unit has an output voltage (the voltage of the output capacitor) of 0 V. When the supply of a high-frequency current to the induction line is started, the constant voltage control circuit of each mobile unit brings the switching device into a full-load condition, which is a continuous opened condition, to charge the output capacitor. This starts control for increasing the output voltage to the reference voltage. In this way, when the supply of a high-frequency current to the induction line is started from the initial condition, all the mobile units receive power in the full-load condition. Thus, the power supply device becomes overloaded and the protective function of the power supply device is activated so as to interrupt power supply to the induction line, thereby stopping all the mobile units. This is because the capacity of power that can be supplied from the power supply device to the induction line is set so as to supply power (power in a normal load condition) consumed by the loads of the mobile units while the output capacitors of all the mobile units are charged (at a constant voltage). If a power capacity exceeding the power of the normal load condition is requested to the power supply device, the power supply device can supply excessive power in a normal condition, increasing the cost of the power supply device.
The same problem occurs also in a momentary power failure of a commercial power supply that supplies power to the power supply device. Specifically, a momentary power failure interrupts power supply from the power supply device to the induction line. Thus, power charged to the output capacitor is supplied to the load in each mobile unit, leading to a rapid decrease in output voltage. When power supply to the induction line is restarted, the constant voltage control circuit of each mobile unit starts control for increasing the output voltage of the full-load condition to the reference voltage. Hence, all the mobile units are placed in the full-load condition and thus the power supply device becomes overloaded, interrupting power supply from the power supply device to the induction line.
Furthermore, the same problem occurs when the power supply to the induction line is temporarily interrupted and then is restarted in an energy-saving operation or at the restart of power supply to the induction line when power supply is repeatedly interrupted and restarted in an intermittent operation.